1. Field of the Invention
The present invention relates to a magnetic sensing element used for a hard disk device, a magnetic sensor, etc., and particularly to a magnetic sensing element having a specular layer with excellent reproducing characteristics, and permitting proper control of the magnetization of a free magnetic layer even with a narrower track, and a method of manufacturing the magnetic sensing element.
2. Description of the Related Art
In order to appropriately comply with track narrowing with recent increases in recording density, a structure for controlling the magnetization of a free magnetic layer by using a so-called exchange bias system is being the mainstream.
It is also known that when a specular layer (specular reflection layer) comprising a Ta oxide is provided for extending the mean free path λ+ of conduction electrons having, for example, up spin, both the rate (ΔR/R) of change in resistance and reproduced output can be improved.
The above-described magnetic sensing element having the specular layer and using the exchange bias system for controlling magnetization of the free magnetic layer is though to be a desired structure for realizing a higher recording density in future.
The structure of a conventional magnetic sensing element and a manufacturing method therefor will be described below. FIGS. 17 and 18 are drawings respectively showing steps for manufacturing a conventional magnetic sensing element. Each of the figures is a sectional view of the magnetic sensing element taken along the side facing a recording medium.
In the step shown in FIG. 17, an antiferromagnetic layer 2 comprising, for example, a PtMn alloy, is formed on a substrate 1, and a pinned magnetic layer 3 comprising a magnetic material, a nonmagnetic material layer 4 and a free magnetic layer 5 comprising a magnetic material are further laminated on the antiferromagnetic layer 2. Furthermore, a specular layer (specular reflection layer) 9 is formed on the free magnetic layer 5. The specular layer 9 is formed by first depositing a Ta film and then oxidizing the Ta film. The Ta film can easily be oxidized by air exposure.
Next, a lift-off resist layer 10 is formed on the specular layer 9 shown in FIG. 17, and the portions of the specular layer 9, which are exposed on both sides of the resist layer 10 in the track width direction (the X direction shown in the drawing), are completely removed by ion milling. In this step, the free magnetic layer 5 below the specular layer 9 is also partially removed (portions shown by dotted lines).
In the next step shown in FIG. 18, a ferromagnetic layer 11, a second antiferromagnetic layer 12 made of a IrMn alloy, or the like, and an electrode layer 13 are continuously deposited on each of the portions of the free magnetic layer 5, which are exposed on both sides of the resist layer 10. Then, the resist layer 10 shown in FIG. 18 is removed.
In the magnetic sensing element shown in FIG. 18, a track width Tw can be defined by the distance between the ferromagnetic layers 11 in the track width direction (the X direction shown in the drawing), and the ferromagnetic layers 11 are strongly pinned in the X direction by an exchange coupling magnetic field produced between the ferromagnetic layers 11 and the second antiferromagnetic layers 12. Therefore, both end portions A of the free magnetic layer 5, which are respectively positioned below the ferromagnetic layers 11, are strongly pinned in the X direction by ferromagnetic coupling with the ferromagnetic layers 11, and the central portion B of the free magnetic layer 5 in the track width Tw region is possibly put into a single magnetic domain state to a weak level permitting reversal of magnetization with an external magnetic field.
However, the conventional magnetic sensing element formed in the manufacturing steps shown in FIGS. 17 and 18 has the following problems:
(1) First, not only the specular layer 9 but also a portion of the free magnetic layer 5 formed below the specular layer 9 are removed during ion milling in the step shown in FIG. 17, and thus an inert gas such as Ar or the like, that is used for ion milling, easily enters the exposed portions of the free magnetic layer 5 from the surface. Therefore, the crystal structure of the surface portions 5a of the free magnetic layer 5 is broken by damage due to the ion milling, or a crystal defect easily occurs in the structure (Mixing effect). Thus, the magnetic characteristics of the surface portions 5a of the free magnetic layer 5 easily deteriorate.
It is most preferable that only the specular layer 9 can be removed without removal of the free magnetic layer 5. However, it is difficult to actually control such ion milling.
A reason for this lies in the thickness of the specular layer 9 formed on the free magnetic layer 5. As described above, the specular layer 9 is formed by depositing the Ta film, and then oxidizing the Ta film.
In the deposition step, the Ta film conventionally functions as an anti-oxidation layer for protecting the free magnetic layer 5 formed below the Ta film from oxidation, and when the Ta film is formed to an excessively small thickness, therefore, the free magnetic layer 5 cannot be appropriately protected from oxidation.
In the deposition step, the Ta film is formed to a thickness of as large as 10 Å or more, preventing the free magnetic layer 5 formed below the Ta film from being oxidized by air exposure.
However, when the Ta film is oxidized by air exposure, the thickness of the oxidized portion increases to increase the thickness of the specular layer 9 formed by oxidizing the Ta film to be larger than the thickness of the Ta film in the deposition step. As described above, when the Ta film having a thickness of 10 Å or more is formed in the deposition step, the specular layer 9 having a thickness of 20 Å or more is formed.
Therefore, in order to effectively remove both end portions of the specular layer 9 by milling in the step shown in FIG. 17, high-energy ion milling is required. High-energy ion milling has a high milling rate, and it is thus nearly impossible to stop milling at the moment when the thick specular layer 9 is completely removed by ion milling. Namely, as the energy increases, the need to provide a wide margin for a milling stop position increases. Therefore, the free magnetic layer 5 formed below the specular layer 9 is partially removed, and is easily excessively damaged by the high-energy ion milling to significantly deteriorate the magnetic characteristics.
(2) As described above, the surface of the free magnetic layer 5 exposed by ion milling is damaged by the ion milling to deteriorate the magnetic characteristics. Therefore, magnetic coupling (a ferromagnetic exchange interaction) with the ferromagnetic layers 11 laminated on the free magnetic layer 5 is not sufficient, and thus the ferromagnetic layers 11 must be formed to a large thickness.
However, when the ferromagnetic layers 11 are formed to a large thickness, exchange coupling magnetic fields produced between the ferromagnetic layers 11 and the antiferromagnetic layers 12 are weakened, failing to strongly fix magnetizations of both end portions A of the free magnetic layer 5 to produce the problem of side reading, and failing to manufacture a magnetic sensing element capable of complying with track narrowing.
When the ferromagnetic layers 11 are formed to an excessively large thickness, an excessive static magnetic field is easily applied to the central portion B of the free magnetic layer 5 from the inner side plane of each of the ferromagnetic layers 11, thereby easily deteriorating the sensitivity of the central portion B of the free magnetic layer 5 to an external magnetic field, the central portion B permitting reversal of magnetization.
As described above, in the structure of the magnetic sensing element in which the specular layer 9 comprising a Ta oxide is formed on the free magnetic layer 5, and the ferromagnetic layers 11 and the antiferromagnetic layers 12 are laminated on the portions of the free magnetic layer 5, which are exposed by removing both end portions of the specular layer 9, magnetization of the free magnetic layer 5 cannot be appropriately controlled, and a magnetic sensing element capable of complying with a narrower track cannot be manufactured.